Direct series electric drive systems for machines typically include a power circuit that selectively activates one or more drive motors at a desired torque. Each of the drive motors is connected to a wheel or other traction device that operates to propel the machine. A direct series drive system also includes a prime mover, for example, an internal combustion engine, that drives a power generator. The power generator produces electrical power that is often conditioned by a power circuit, and ultimately used to drive the motor. Conceptually, as the machine is propelled, mechanical power produced by the engine is converted into electrical power by the generator. This electrical power may be processed and/or conditioned by the power circuit before it is metered to the motors. The motors transform the electrical power back into mechanical power that drives the wheel and propels the machine.
Heavy machinery, such as off-highway truck and wheel loader equipment, is commonly used in mining, heavy construction, quarrying, and other applications. The adoption of electric drive systems has been viewed as improving efficiency and reliability of such machinery. Electric drive systems generally require less maintenance and thus, have lower life cycle costs.
Earlier generations of power systems for driving multiple electric motors relied upon a single generator to supply power to a single power bus. Multiple motors, in turn, are powered via the single power bus. More recently power systems have been proposed wherein multiple generators supply power for powering the multiple motors. Alster et al., U.S. Pat. No. 7,152,705 proposes an electric drive arrangement where generators, driven individually by dual engines, charge a single power storage module. The power bus for a set of electric drive motors, in turn, is powered directly by the power storage module.
Another example of a multiple generator system for driving multiple electric motors divides power supply into a distinct power bus for each generator. In the described embodiment, a first generator provides power to a first power bus connected to a first set of two motors. A second generator provides power to a second power bus connected to a second set of two motors.
Yet another multi-generator system is described in Wiegman et al., U.S. Pat. No. 7,319,307. In this instance, a master bus power control unit measures the total alternating current (AC) output power of multiple synchronous generators connected to a same AC power bus. The master bus power control unit exercises supervisory control over the individual ones of the multiple synchronous generators by issuing correction or adjustment signals to field control units associated with individual ones of the generators. As such, the master bus power control unit uses total power supplied by multiple generators on a single AC power bus to render control decisions and subsequently issue control signals to individual field control units for the generators.
Multiple generator power systems offer advantages over single generator power systems. For example, smaller generators can be used that are potentially less expensive than a single generator providing twice the power. Moreover, a variety of partial failure modes are potentially supported in the event that one of the power sources fails. Such benefits are presented in the examples of known systems mentioned above.